The term volunteer bias refers to a specific bias that can occur when the subjects who volunteer to participate in a research project are different in some ways from the general population. If this occurs, the researcher has sampled only a subset of the population, and consequently, the data gathered are not representative of all people, merely of those that choose to volunteer. Volunteer bias is a challenge to the external validity of any research project.
An underlying problem in volunteer bias is that volunteers and nonvolunteers are different in important ways. In an extensive review, Robert Rosenthal and Ralph Rosnow examined differences between volunteers and nonvolunteers. They reported that, in general, volunteers are more educated, come from a higher social class, ...
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